They might even break off a bolt. Bores in the soft material of aluminium vehicle engine blocks or cylinder heads are particularly vulnerable. While buying a replacement bolt may be no big deal, scrapping a larger component like an oil sump or cylinder head because of thread issues might prove prohibitively expensive. Hence the development of a range of quick fixes.
Since the 1950s, engineering solutions have been available to fix broken threads (main image) and broken bolts, without scrapping the part.
“All aeroplanes flying today probably have Helicoils fitted,” states Clive Brown, sales manager, Bollhoff (01902 603934) speaking about the Helicoil wire coil insert, originally marketed to aerospace as the ‘Aerothread’. In its traditional form, the diamond-shaped stainless steel wire coil consists of three main parts: the coil of the insert, the tang, which is a coil on the end used to drive the insert in, and a notch just short of the end of the tang (pictured, below right). Once the insert is installed, the tang can be broken off. The gaps in between the strands of wire on the inside of the insert mesh perfectly with the thread of a bolt.
Brown explains how to install a Helicoil: drill out the existing threads, then tap the hole with a proprietary tap that is slightly larger than standard to accommodate the size of the insert. The insert is installed by being pre-wound on a special installation tool which reduces the coil diameter so the insert winds off the tool and into the thread of the hole, and then the installation tool is backed out. At that point the spring relaxes as much as the bore allows, pressing it against the walls, so does not come out of the hole.
That’s the general picture, although Helicoils come in coated or plated versions, and other modifications launched alter the design (Helicoil tang free, pictured at right) or introduce methods of automated installation.
In addition, there are other types and brands of inserts that aim to accomplish the same thing. ‘Recoil’ inserts are from Australia and are widely available from UK stockists. Alternatives made not from wire but solid material include ‘Ez-Lok’, a brand from the US distributed by AMA Fasteners (075281 10252) and the Time-Sert from Wurth (03300 555 444). Both Ez-Lok and Time-Serts resemble a thimble with exterior threads. On the top of the former is a slot on the top for winding down; on the latter is a flange that is used to seat securely on the top of the surface above the hole. For this reason, installation of Time-Serts includes an extra step of drilling a chamfer around the top of the hole to accommodate this feature. As the Time-Sert is installed, its lower threads deform and are pressed outwards into the material as the special installing tool similar to a tap progresses down its length, not unlike the action of a plastic insert used to fasten a screw into a hole drilled in a masonry wall.
Drilling, tapping, using a die: these metalworking skills are required to fix threads, and may be outside the comfort zone of many of the maintenance technicians facing such problems. “A lot of people are scared of drilling. A lot of them drill way too fast, and then they end up snapping the bolt as the drill goes through. I think that’s what scares them,” observes Clinton Herholdt, proprietor of Essex-based bolt repair company Threadworx (07713 902011). But in fact he says: “It’s not rocket science; it’s just taking your time. On certain jobs there’s a sequence, a certain way of doing it, and you get used to doing it all day every day.” Herholdt has been doing it for seven years in the UK – and before that another 20 years overseas.
He admits his first port of call for a broken bolt is welding on an extension, and then turning that. He also uses Ez-Out bolt extractors (widely available), or a variety of other standard tools, such as a pencil grinder with tungsten bit, to take out broken drills, as well as some he makes himself.
Helicoil inserts are a firm favourite of Herholdt, which is not to say they are he doesn’t attend jobs where someone has broken one. He explains: “I had one the other day where the guy tried to put a Helicoil which was 12 by 1.5 [pitch] underneath the subframe of his VW Golf, and he didn’t check that it was a 1.5 thread pitch. He bought a standard Helicoil, which is 12 by 1.75 pitch. And he tapped it, without realising it was a different thread, and he screwed the Helicoil in. When he undid the bolt, it came out on the bolt, because it obviously wasn’t lining up properly. That’s the most critical thing. Before you do any thread repair, pick up the original bolt and have a look at it and make sure you have got your threads right. The standard 6mm thread has a pitch of 1.0; 8mm pitch is 1.25; 10mm is 1.5 and 12mm is 1.75. That’s standard. But you do get an 8 by 1, 10 by 1.25 and 12 by 1.5 pitch –which they call ‘fine thread.’”
His solution for this customer was to fabricate a custom sleeve with an internal thread profile of 12 by 1.5 and an external diameter of 16 by 1.5. “There are certain jobs that I do where you can’t buy an insert for that. Or I need an insert to be longer than they supply, because there’s a lot of torque on that, and I’ll actually make one on my lathe and go round and fit it.” Similarly, he has no compunctions about cutting down inserts to fit thinner workpieces, down to a minimum of 3-4 threads.
Another quirk of fastening to look out for are stretch bolts, used sometimes on vehicle cylinder heads, he advises. “If it’s got a degree torque, it’s designed to be a stretch bolt, which means when you tighten it, for example, you can have a thread where your torque settings are 40Nm plus 30º plus 60º.” Tightening the bolt causes it to stretch, which alters the pitch of the thread and stops the bolt coming loose.
He also recommends taking care to protect inner volumes, such as engine cylinders, from swarf and debris produced during the process of fitting an insert. In the case of spark plugs in an engine, one method is sticking the finger from a rubber glove down the hole to stop any falling debris. Another method is to raise piston up on that cylinder, make sure the engine is firing and that the valves are closed; then there is nowhere for the swarf to go except outside the spark plug hole. Herholdt adds: “A lot of the guys [vehicle technicians] don’t know that trick. They tap it and try to vacuum the stuff out. But if you get a piece of aluminium or swarf stuck underneath the valve, that creates a miss. So you put your new plug in, you start your car and that cylinder won’t fire. Then you need to pull the head off.”